Magnetic resonance imaging (MRI) is an extremely powerful technique for visualizing tissue, particularly soft tissue, of a patient. The technique relies on exciting nuclei, typically hydrogen nuclei, from their equilibrium state, and measuring the resonant radio-frequency signals emitted by the nuclei as they relax back to equilibrium.
In an MRI procedure the cardiac condition of a patient placed in the rapidly changing magnetic fields generated by the procedure may need to be assessed. Monitoring of an electrocardiograph (ECG) signal is a good indicator of the cardiac condition of the patient, so that ECG monitoring may enhance the efficacy of the procedure.
U.S. Pat. No. 7,039,455, to Brosovich, et al., whose disclosure is incorporated herein by reference, describes apparatus for improving the quality of ECG signals obtained from a patient undergoing MRI. The apparatus includes the arrangement of a differential amplifier, a prefilter, a signal limiter circuit and an intermediate amplifier with an integral low pass filter.
U.S. Pat. No. 7,286,871, to Cohen, whose disclosure is incorporated herein by reference, describes a method of reducing contamination of electrical signals recorded in the presence of repeated interference contamination The electrical signal is digitized, the digitizing beginning with a timing signal. A plurality of digitized electrical signals is analyzed, and the electrical signals are synchronized with respect to the timing signal, to obtain an estimated contaminating signal that is subtracted from the digitized electrical signal.
U.S. Pat. No. 4,991,580, to Moore, whose disclosure is incorporated herein by reference, describes a method for improving the quality of ECG signals obtained from a patient undergoing MRI. The method includes conducting the ECG signals having MRI induced noise signals to the input of a slew rate limiter (SRL) circuit having a preselected maximum slew rate. The output of the SRL circuit is connected to a low pass filter circuit.
U.S. Pat. No. 6,070,097, to Kreger, et al., whose disclosure is incorporated herein by reference, describes a method for generating a gating signal for cardiac MRI. An MRI system includes a detector system which receives an ECG signal from a patient being scanned and produces the gating signal. The gating signal is produced when a detected peak in the ECG signal meets a set of R-wave criteria.
European Patent 1,872,715, to Uutela Kimmo, whose disclosure is incorporated herein by reference, describes reference data indicative of statistical properties of biosignal artifacts that is generated by a turbulent electromagnetic environment and that is collected. Multiple channels of the biosignal of the patient are measured in the turbulent electromagnetic environment. Artifacts in the multiple channels are detected using the reference data and parameters are derived for a linear combination of the multiple channels. A refined biosignal is obtained by applying a linear combination to desired signal samples of the multiple channels, the linear combination being defined by the parameters derived.
PCT application WO/2012/170119, to Schweitzer et al., whose disclosure is incorporated herein by reference, describes a system for tracking catheter electrode locations with the body of a patient during an MRI scan sequence The system includes mitigation logic configured to identify one or more impedance measurements that were taken during potentially noise-inducing conditions, and were thus subject to corruption by noise. The mitigation logic is configured to replace the potentially corrupt impedance measurements with previously-obtained impedance measurements taken from an immediately preceding acquisition cycle.
Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.